How should I use this to measure whether or not the nut slot needs filing? Thanks.

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Yes.

The general idea of the Nut Slot Height feature is to be able to project a perfectly straight line from the bottom of the nut slot, directly to the top of the second fret - ideally leaving a slight gap above the 1st fret as the gauge passes over the 1st fret.

The purpose of the "thin corner" on the Nut Slot Height feature, is to allow the gauge to (hopefully) slip all the way down to the bottom of the narrowest slot (high e string) to give an accurate reading.

For every string, there's an ideal height for each nut slot - that's slightly higher than if there was just another fret where the nut is now.

Not a ton of difference - Actually less than the thickness of a thin sheet of paper on the high e string.

If the nut slots are even a few thousandths (.001") too low, you'll be forced to compensate by adding more relief (i.e. loosening the truss rod), or raising the saddle(s)

Starting with the low E string, place the gauge corner into the low E slot, then the other end of the gauge across to the 2nd fret.

Now look at the gap between the bottom edge of the gauge and the 1st fret.  You should be seeing some daylight.

How much?  I'll give you some target numbers that are if anything just slightly high - and depending upon your particular guitar, you can work each slot down the next few thousandths through testing with strings on.

You can use feeler gauges when measuring between the gauge and the 1st fret, or even various thicknesses of paper. I've used both with equally good results.

Between a regular sheet of cheap/thin copy paper (.0035") and a standard mailing envelope (the kind that bills come in - .005")...  there's few combinations you can't create by either double-folding a strip, or combining one of each. These combinations coincide pretty closely with the low E to high e measurements.

High e setting is generally not more than .003".  When using paper, that's a snug fit - you'll feel some drag when pulling the paper between the fret and the gauge.

Low E setting is (almost) never more than .009" (typically .006” - .008").

Every other string is spread fairly incrementally between those points.

Below I'll give each string and the typical range of measurement:
String:

- E - .006” - .009"
- A - .006" - .008"
- D - .005" - .006"
- G - .004" - .006"
- B - .003" - .005"
- e - .0025" - .0035"

The only (slight) variation tends to be the unwound G on an electric...  often you'll leave it as high as the D string. As low as possible without buzzing n the open position is the goal, so notes don't pull sharp in the first few frets.

So, how do you know when you’ve REALLY got this dialed in?

When you feel like you’ve got the action and relief about as good as you can adjust it...

What you should shoot for (at least what I look for) is to make every open string note just slightly harder to make buzz than it is on the lowest fretted notes. Often giving a very slight allowance on the open E string if you really like to dig in for a big E chord.

Item Weight	36 g
Package Dimensions	10.19 x 7.98 x 1.19 cm; 36 g
Back Material	Metal
Body Material	Metal
Color Name	Metric
Fretboard Material	Polished Wood
String Material	Stainless Steel
Neck Material Type	Metal
Number of Strings	6
Guitar Bridge System	Fixed

FretGuru Fret Rocker String Action Gauge Guitar Tool, Metric | Professional-grade setup tool, Ultra-Precision measurements for best guitar playability, CNC machined, diamond honed polished

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